A. Field of the Invention
The present invention relates generally to network data transmission, and more particularly, to data transmission through cable modem systems.
B. Description of Related Art
Cable modems allow end-users to connect to networks, such as the Internet, through cable TV lines. In a manner similar to traditional telephone modems, cable modems modulate between digital signals from an attached computer to analog signals that are transmitted over the cable lines. Unlike traditional telephone dial-up modems, however, cable modems may provide significantly greater throughput.
Cable modems are generally installed locally to the end-user, and communicate with a cable modem termination system (CMTS) at a local cable TV company office. Multiple cable modems may share a single communication channel with the CMTS. The cable modems can receive from and send signals to the CMTS, but not to other cable modems on the line.
When sharing a communication channel with a CMTS, the cable modems may use a time division multiple access (TDMA) scheme in which the modems transmit units of data, called frames, to the CMTS only during designated time intervals. The CMTS receives these frames as a series of intermingled frames and separates the frames into a series of time sequenced frames based on the transmitting cable modems.
FIG. 1 is diagram illustrating transmission and reassembly of a series of frames from two different cable modems. The first cable modem, labeled as cable modem 101 (cable modem X), wishes to transmit the series of frames X1, X2, and X3. The second cable modem, labeled as cable modem 102 (cable modem Y), transmits the two frames Y1 and Y2. Cable modems 101 and 102 transmit their respective frames over a shared cable line to CMTS 103. As shown, the frames are intermingled in time. CMTS 103 reconstructs the order of frame transmission from cable modems 101 and 102 based on information in the header of each frame that identifies the cable modem associated with the frame. A correctly reconstructed series of frames from cable modems 101 and 102 is shown in FIG. 1 as reconstructed X frames 104 and reconstructed Y frames 105.
Data Over Cable Service Interface Specification (DOCSIS) is a commonly used cable modem protocol that defines interface requirements for cable modems. Under DOCSIS, cable modems, instead of transmitting a complete frame, may transmit only a portion of a frame, called a frame fragment. The CMTS receives the frame fragments and reassembles the frame fragments into complete frames. As with entire frames, frame fragments may arrive at the CMTS intermingled with frames or frame fragments from other cable modems.
FIG. 2 is a diagram illustrating fragmented frames. Cable modem 101 again transmits three frames (X1, X2, and X3). Cable modem 102 transmits two frames, the first of which (Y1), is broken into three frame fragments, labeled as Y1(1), Y1(2), and Y1(3). The frame fragments may be intermingled with the frames from cable modem 101. The CMTS, as well as ordering the received frames based on the transmitting cable modem, reassembles frame fragments into complete frames, such as complete frame 210.
In high bandwidth applications, in which multiple cable modems communicate with a single CMTS, it is desirable to reassemble frame fragments and order frames in the correct frame sequence in an efficient manner.